Microscale out-of-plane anemometer

ABSTRACT

A microscale out-of-plane thermal sensor. A resistive heater is suspended over a substrate by supports raised with respect to the substrate to provide a clearance underneath the resistive heater for fluid flow. A preferred fabrication process for the thermal sensor uses surface micromachining and a three-dimensional assembly to raise the supports and lift the resistive heater over the substrate.

PRIORITY CLAIM

This application claims priority of U.S. Provisional Application Ser.No. 60/349,431, filed Jan. 18, 2002 under 35 U.S.C. § 119.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government assistance under NationalScience Foundation Grant No. NSF IIS 99-84954, National Aeronautics andSpace Administration Grant No. NASA NAG 5-8781, and National ScienceFoundation Grant No. NSF IIS-0080639. The Government has certain rightsin the invention.

FIELD OF THE INVENTION

A field of the invention is sensing. Other fields of the inventioninclude microelectronics and micro-electromechanical systems.

BACKGROUND OF THE INVENTION

Measuring fluid flow velocity is useful for turbulence flow measurementin fluid mechanics research, and any industrial application where flowmeasurement is required, such as, but not limited to, gas metering andair duct monitoring. Commercial flow sensors are mainly based on one oftwo principles: thermal anemometry and laser-Doppler velocimetry. Athermal anemometer is a common type of commercial flow sensor formeasuring the velocity of fluid flow. A typical type of thermalanemometer, often referred to as a “hot-wire anemometer”, utilizes aresistive heater (a “hot wire”) that serves as both a Joule heater and atemperature sensor. Monitoring the resistance of the resistive heater ascurrent is passed through determines the temperature of the element.

Under a constant bias power and zero flow rate, the temperature of theresistive heater reaches a steady-state value. As flow of a fluid mediapasses the resistive heater, heat is transferred from the element to thefluid media via forced convection, thus reducing the temperature of thesensor. The flow speed is derived indirectly from the temperaturevariation from steady state values. Accordingly, the temperature of theresistive heater provides a means to gauge the cooling rate of theelement and the flow velocity.

A conventional hot-wire anemometer includes a thin wire made of platinumor tungsten that is supported by prongs and mounted on a probe having asuitable electrical connection. This thermal sensor provides a fastresponse (in the kilohertz range), with low noise. The sensor also canbe made relatively small and inexpensively.

However, conventional hot-wire anemometers suffer from significantshortcomings. One such shortcoming is that the fabrication process isdelicate and may not result in uniform performance. Another problem isthat it may be prohibitively difficult to form large arrays of theanemometers for measuring flow distribution, for example.

Micromachined anemometers have been used by those in the art to realizea thermal sensor with smaller dimensions, better uniformity, fasterfrequency response, and lower cost of production (via the batchprocessing nature of micromachining, for example). They also provide theability to perform applications such as, but not limited to, flow fieldmeasurement. Conventional micromachined anemometers have been producedusing a bulk micromachining technique, resulting in free-standingcantilevered structures within substrates. For example, dopedpolycrystalline silicon may be used to make prongs and resistive heatersby bulk micromachining. To create a significant distance between theresistive heater and the substrate, thus increasing thermal insulationto the substrate and increasing the sensitivity, the cantileveredstructures are formed by at least partially removing the siliconsubstrate.

However, bulk micromachining incurs significant cost and restricts thetype of substrate that can be used. For example, the doping of silicon(to create the resistive heater, for example), the etching of thesilicon, and the packaging of individual silicon dies requiresignificant expertise and effort. Additionally, most micromachinedhot-wire anemometers cannot be realized effectively in a large arrayformat. Furthermore, bulk micromachining requires significant etchingtime, and bulk etching using anisotropic wet etchants frequently posesconcerns of materials compatibility, as all materials on a givensubstrate are required to sustain wet etching for long periods (severalhours to etch through typical silicon wafers, for example).

Also, many other types of devices use doped polysilicon thin film as thematerial for the resistive heater. However, polysilicon deposition andannealing require a high-temperature process and generally preclude theuse of substrates with a low melting point.

Certain microscale hot-wire anemometers use surface micromachining for asimpler fabrication process. However, the resistive heater employed istypically located directly on the substrate or very close to it. Thisleads to a slower frequency response and reduced sensitivity.

SUMMARY OF THE INVENTION

The present invention provides, among other things, a thermal sensor ona substrate for measuring fluid flow. The thermal sensor includes aresistive heater suspended above the substrate by supports. The supportsare connected to the substrate, either directly or indirectly, and areraised at a sufficient angle with respect to the substrate to create aclearance between the resistive heater and the substrate for fluid flow.The substrate may be of various materials, and is not limited to asilicon substrate. Furthermore, the thermal sensor is not limited todoped polysilicon, but rather can include a metal thin film.

A method of fabrication for a thermal sensor on a substrate is alsoprovided. A sacrificial layer is formed on the substrate, and a metalthin film is patterned to form a sensing element. At least one supportfor the sensing element is patterned. The sacrificial layer is removed,and the sensing element is lifted away from the substrate by raising thesupports, thus creating a clearance between the sensing element and thesubstrate to allow fluid flow between the sensing element and thesubstrate. The supports are raised preferably by use of a magnetic fieldapplied to magnetic material patterned on the supports.

Additionally, a chip for measuring fluid flow is provided according tothe present invention. The chip includes a substrate, a thin film metalhot-wire sensor, and a pair of supports raising the thermal sensor andsuspending the sensor over the substrate to create a clearance betweenthe sensor and the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a preferred embodiment thermalsensor disposed on a substrate;

FIGS. 2A–2I are schematic side views showing steps in a preferredprocess of fabricating a thermal sensor on a chip;

FIGS. 3A–3C are schematic side views showing steps in a preferredprocess of electroplating a bending region of a thermal sensor;

FIG. 4 is a scanning electron micrograph (SEM) image of a thermal sensorwith an electroplated bending region;

FIG. 5 is an SEM of an alternative thermal sensor without polyimidesupports;

FIG. 6 is a diagram of a fluid flow measurement system including athermal sensor;

FIG. 7 is a schematic diagram of a circuit for constant current testingof a thermal sensor;

FIG. 8 is a schematic diagram of a circuit for constant temperaturetesting of a thermal sensor; and

FIG. 9 is an SEM of an array of thermal sensors on a substrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides, among other things, a microscale thermalsensor produced on a substrate by surface micromachining. The thermalsensor provided on a substrate includes a resistive heater, preferablycontaining a thin film, suspended above the substrate by supportsattached at one end to the substrate, either directly or indirectly. Thethin film is produced from temperature-sensitive, electricallyconductive material. As opposed to conventional microscale anemometers,the present thermal sensor allows non-silicon materials to be used forthe resistive heater and/or the substrate. Furthermore, it is preferredthat the resistive heater including the thin film be made of anon-silicon material, such as a metal.

The supports are raised at an angle with respect to the substrate tosuspend the resistive heater, preferably creating a clearance underneaththe resistive heater for flow of a fluid media. The clearance may bedefined, for example, by the resistive heater, the substrate, and thesupports. The supports preferably suspend the resistive heater atopposing ends of the element.

The supports preferably include cantilevered prongs formed from aductile metal beam, such as gold, so that the support can bend about abending region or hinge, plastically deforming when lifted so that thesupport remains in a raised position after the lifting processconcludes. The supports preferably further include a polymer structuralsupport, such as a polyimide support beam, to provide structuralrigidity to the supports. If the metal beam is conductive, the supportscan also serve as electrical leads for the resistive heater.

The film for the resistive heater preferably includes metals such asnickel and/or platinum. A polyimide structural support layer may alsooverlap the film and form part of the resistive heater, providingstructural support for the resistive heater. A chrome layer may also bepresent to adhere the polyimide layer to the film. The resistive heatermay have various shapes, including a straight line, wave-like shapes(square wave, for example, or tooth), etc.

Because the resistive heater is preferably raised away from theimmediate velocity boundary layer next to the substrate, higher flowspeed and convection heat-transfer rate is experienced by the resistiveheater. This improves the sensitivity and response time of the thermalsensor. Additionally, the raised thermal sensor provides increasedthermal insulation from the substrate.

The types of substrates on which the thermal sensors are produced mayvary, due at least in part to the surface micromachining fabricationprocess. Surface micromachining also enables more efficient assembly andallows formation of large arrays of thermal sensors. Furthermore,because the resistive heaters are preferably realized using non-siliconmaterials, the fabrication process can be realized in a more efficientand less costly manner.

Three-dimensional assembly methods are preferably used in conjunctionwith the surface micromachining to produce the thermal sensors of thepresent invention. This circumvents the use of bulk micromachining. Thepreferred three-dimensional assembly method pairs magnetic actuationwith deformable metal hinges of the supports to fabricate a thermalsensor using surface micromachining of preferably metal and polymermaterials. The magnetic actuation preferably includes application of amagnetic field to a ferromagnetic layer formed on the thermal sensor.

Preferably, the maximum temperature required throughout the process flowis under 350° C., but this temperature may vary, and may be even lower,such as, but not limited to, below 200° C. By limiting the overallprocess temperature, the preferred fabrication process can be run on abroad range of substrates including, but not limited to, silicon, glass,and plastics. The chosen substrate's glass transition temperature(T_(G)) should be higher than the maximum processing temperature.Otherwise, the substrate choices may vary widely.

It is also preferred that no etching using concentrated hydrofluoric(HF) acid be necessary. This is beneficial for at least the reason thatconcentrated HF (a commonly used sacrificial-layer etching solution insilicon-based surface micromachining) is prone to attack the interfacebetween a hot wire and its support structure. Heretofore, solutions tothis problem have added significant process control complexity.

Referring now to the drawings, FIG. 1 shows a preferred embodimentthermal sensor 10. The thermal sensor 10 includes a resistive heater 12that is elevated; i.e., suspended, above a substrate 14, supported by aplurality (as shown, two) of supports 16. Preferably, the height of theresistive heater 12 above the substrate 14 is predetermined, andcorresponds (though not necessarily equates) to the length of thesupports 16. The predetermined support length and therefore theresistive heater 12 height above the substrate is preferably decidedaccording to, for example, the type of flow (laminar vs. turbulent, forexample), velocity of flow, roughness of the surface, etc. As shown inFIG. 1, the supports 16 are raised at an angle θ to the substrate 14,preferably between 10° and 170°, and most preferably about 90°, toelevate the resistive heater 12. Other angles for the supports 16between 0 and 180° are possible as well, by changing the orientation ofthe magnetic field. By elevating the resistive heater 12 away from thebottom of a velocity boundary layer of the substrate 14, the resistiveheater experiences greater fluid flow velocity, as shown in flowvelocity profile 18, and exhibits greater sensitivity. The raisedresistive heater 12 and the substrate 14 (and preferably the supports 16as well) collectively define a clearance 20 for fluid flow, allowing afluid media (illustrated at 22) to flow through the clearance, as wellas above the resistive heater.

The supports 16 are preferably in the form of first and secondcantilevered prongs 16 attached at one end such as a base 24 to thesubstrate 14, supporting the resistive heater 12 at first and secondopposing ends 26 of the element. The first and second prongs 16preferably include a metal beam 28 (see FIG. 2I), such as, but notlimited to, gold or aluminum, for electrical conduction and mechanicalresiliency for raising the thermal sensor 12 from the substrate 10.Preferably, any ductile metal that conducts electricity may be used, aslong as the resistance of the filament of the resistive heater 12 issignificantly (a few orders of magnitude, for example) larger than themetal beam 28. Each of the first and second prongs 16 may, but notnecessarily, also include a polyimide support beam 30 for mechanicalsupport of the prongs. Preferably, the resistive heater 12 iselectrically coupled to other components through the prongs 16, so thatthe prongs provide both a mechanical and electrical connection for theresistive heater. Conductive paths may be created from the prongs 16 toother components during fabrication.

Pads 32, of any suitable material or materials with good adhesion to thesubstrate 14, such as, but not limited to, gold and chrome, arepreferably provided at or near the base 24 of each of the prongs 16 forproviding mechanical support for the prongs to the substrate 10, toaccount for shear stress due to fluid flow. A bending joint 34 iscreated for each of the supports 16 near the base 24 of the supports,where the supports begin to raise from the substrate 14. The bendingjoints 34 may be reinforced by electroplating nickel, for example, toenhance the rigidity of the thermal sensor 10.

As opposed to the doped polysilicon hot-wire elements of conventionalanemometers, the resistive heater 12 of the present invention may andpreferably does include a non-silicon material such as, but not limitedto, a thin film metal 40 (FIG. 2I) or filament, having a hightemperature coefficient of resistance (TCR). A polymer piece 42, such asa polyimide support beam, may also be present to provide additionalstructural integrity to the resistive heater 12. Also, at least in partbecause the thermal sensor 10 is produced using surface micromachining,the thermal sensor may be implemented on various substrates 14.Particularly, a non-silicon substrate 14 is used, including but notlimited to flexible and/or curved substrates. However, the lowprocessing temperature available and preferably used in fabrication ofthe thermal sensor 10 according to a preferred method allows the thermalsensor to be built on top of an integrated circuit substrate as well,without affecting the transistor property of the circuit.

A hot-wire anemometer such as the thermal sensor 10 of the presentinvention operates by sensing temperature change of the resistive heater12 resulting from forced convection. The temperature variation can beinferred by the change of the resistance of the thin film 40 as acurrent i (FIG. 1) passes through the supports 16 and the resistiveheater 12. A suitable current source 76 (FIG. 6) may be provided tosupply the current i to the resistive heater 12. The heat balanceequation for the resistive heater 12 under electrical Joule heating isQ _(s) =Q _(gen) −Q _(conv) −Q _(cond),

where Q_(s) is the rate of heat storage, Q_(gen) is the generated (bias)power from Joule heating, Q_(conv) is the rate of heat loss due toforced convection, and Q_(cond) represents the sum of conductive losses(e.g., through the supports). For a hot-wire anemometer, the termQ_(cond) involves both loss from the ends 26 of the resistive heater 12through the supports 16, as well as longitudinal thermal conductionalong the resistive heater. Under a given set of bias power (Q_(gen))and fluid flow rate, it is important to maximize Q_(conv) whileminimizing Q_(cond) in order to obtain greater sensitivity to velocitychanges. To minimize the conductive heat loss also means that thethermal sensor 10 can be operated in a more thermally efficient manner.This is especially important if an array of sensors is used.Accordingly, the thermal sensor 10 of the present invention reduces theend loss by supporting the resistive heater 12 using supports 16 raisedout-of-plane that have a high aspect ratio and a small cross-section,creating a relatively large thermal resistance. The range of aspectratios and cross-sections may vary, as there is a design trade-offbetween thermal insulation and mechanical rigidity. Furthermore,regarding thermal resistance, a trade-off also exists between electricaland thermal resistance.

One significant design parameter is the length of the resistive heater12. It is possible to increase Q_(conv) by increasing the length of theresistive heater 12, because Q_(cond) varies slowly with the elementlength, whereas the value of Q_(conv) changes roughly linearly withrespect to the length. However, the length of the element 12 may belimited by fabrication practicality and yield considerations. The longerthe resistive heater 12, the more difficult it may be to realize.

Another design parameter is the diameter of the cross-section of theresistive heater 12. In the case that the resistive heater 12 has arectangular cross-section, the equivalent diameter that yields thecross-sectional area may be considered. By reducing the value of thecross-section, the surface-to-volume ratio of the resistive heater 12 isincreased, thus encouraging more convection while confining theconductive component. However, there exists a practical limit to theminimal diameter of the resistive heater 12 as well, due to mechanicalrigidity concerns.

In particular embodiments of the thermal sensor 10, devices includeresistive heater 12 lengths of 50 μm, 100 μm, 150 μm, and 200 μm,preferably between about 10 μm and on the order of millimeters, andheights of the supports 16 of up to on the order of a few millimeters. Apreferred embodiment of the resistive heater 12 is a1200-angstroms-thick metal thin film 40 (but preferably within a rangeof hundreds of angstroms (so that the film exhibits a sufficient TCR) upto several microns thick) overlapping with the polyimide piece 42,though other polymers may be used. A preferred cross-section of thepolyimide piece 42 is 6 μm wide by 2.7 μm thick. The polyimide piece 42provides additional mechanical support for the effective thin film metal40. If the polyimide piece 42 thickness is much lower than 2.7 μm(though it is possible), the mechanical rigidity will likely bedegraded. However, if the thickness is too great, the polyimide beam 42may decrease the frequency response of the thermal sensor 10 due toadded thermal mass.

According to a preferred embodiment thermal sensor 10, the cross-sectionof the resistive heater 12 is comparable to that of commerciallyavailable hot-wire sensors. The surface micromachining process of apreferred fabrication method of the present invention allows goodcontrol of the dimensions of the resistive heater 12.

A preferred method for fabricating the thermal sensor 10 of the presentinvention will now be described with reference to FIGS. 2A–2I. Amaterial for the metal thin film 40 is chosen that provides a hightemperature coefficient of resistance (TCR). A preferred primarymaterial for the metal thin film 40 of the resistive heater 12 isnickel, as it has an effective TCR as a deposited thin film. Thoughother materials, such as platinum, may be used, these materials mayexhibit a reduced effective TCR due to, among other things, ahigher-than-expected electrical resistivity attributable to electronscattering at the grain boundary. Though the boundary scattering can bemodified by, for example, annealing to increase grain size, theannealing temperature in some cases (for Pt, above 600° C.) may beincompatible with integrated circuits and/or the polyimide film 42 usedwith the metal thin film 40 in the preferred thermal sensor 10.

The resistive heater 12 includes the temperature-sensitive non-silicon(preferably metal) thin film 40 preferably overlapped by the supportingpolymer piece 42. The preferably polyimide polymer piece 42 is used forat least the reason that it provides the resistive heater 12 withstructural rigidity without significantly increasing cross-sectionalarea and thermal conductivity. The thermal conductivity of polyimide isalmost two orders of magnitude lower than that of a metal such asnickel.

A preferred fabrication process uses a three-dimensional assembly methodthat utilizes a surface micromachined structure (such as the thermalsensor 10) anchored to substrates with cantilever beams made of ductilemetal materials (for example, gold and aluminum). The ductile metalcantilever beams are the supports 16 in the present thermal sensor 10.Pieces of electroplated ferromagnetic material, such as Permalloy, areattached to the supports 16. By applying an external magnetic fieldH_(ext), the ferromagnetic material is magnetized and interacts with thefield, producing a torque to bend the supports 16 out-of-plane withrespect to the substrate 14. Once the supports 16 are sufficiently bent,the cantilever bending joints 34 are plastically deformed, resulting inpermanently bent supports even after the magnetic field H_(ext) isremoved. This magnetic assembly process may be realized in parallel onthe wafer scale, for example.

Referring now to FIG. 2A, the process begins with a starting wafer forthe substrate 14 such as silicon, glass, polymer, etc. Because thepreferred process has a relatively low overall temperature, the processcan be performed on substrates 14 in addition to silicon. For example,the thermal sensor 10 may be formed on a flexible polymer substrate orother flexible, even curved, substrates for conformal coating of fluiddynamic surfaces of interest.

A sacrificial layer 60 is evaporated and patterned on the substrate 14.Preferably, a chrome/copper/titanium stack is used for the sacrificiallayer 60. More precisely, a chrome film (about 10-nm thick) preferablyserves as an adhesion layer between the remainder of the sacrificiallayer 60 and the substrate 14. A titanium (about 250-angstroms thick)thin film of the sacrificial layer 60 reduces in-process oxidation of acopper film, which itself is preferably about 2500-angstroms thick.Other materials for the sacrificial layer may be used, for example,aluminum, so long as the material chosen can withstand the processtemperature chosen and the etchant of the sacrificial layer does notsignificantly affect the other materials.

Next, as shown in FIG. 2B, a preferably photo-definable polymer 62 suchas polyimide is spun-on, preferably about 2.7 μm thick, patterned vialithography, and cured, for example at 350° C. for about two hours. Thepolymer layer 62, as previously described, forms part of the supports16, including the base 24 and raised prongs (defined by, for example,the portion covering the sacrificial layer 60) and part of the resistiveheater 12.

A Cr/Pt/Ni/Pt film 64 is then evaporated and patterned to complete thethin film 40 of the resistive heater 12 (FIG. 2C). A preferred thicknessof the Cr layer (an adhesion layer) of the film 40 is about 200angstroms. For the remainder of the thin film 40, an 800-angstrom-thicknickel film forming the resistor is sandwiched between two opposing200-angstrom-thick platinum films, which are used to reduce possibleoxidation of the nickel resistor while in operation because platinum isrelatively inert at high operation temperatures. However, other high TCRmaterials may be used for the thin film.

Next, a Cr/Au film 66, preferably about 5000-angstroms thick isevaporated and patterned (FIG. 2D). The Cr/Au film 66, as part of thesupports 16, serves as a mechanical bending element as well aselectrical leads of the resistive heater 12. Other ductile, conductivemetals may be used. A ferromagnetic material 68, such as Permalloy, isthen electroplated (preferably, about 4 μm-thick), using a photoresistmold 70, on portions of the cantilever support prongs 16 formed by theCr/Au film 66 (FIG. 2E). The photoresist is removed (FIG. 2F), leavingthe bending joint 34 exposed for the three-dimensional assembly process.

In one embodiment, and referring to FIG. 2H, a release of thesacrificial layer 60 is then performed by using a solution containingacetic acid and hydrogen peroxide, for example, to selectively removethe copper (adhesive) thin film of the sacrificial layer. Next, amagnetic field H_(ext) is applied, for example by a permanent magnet(preferable field strength of 800 Gauss) at the bottom of the substrate14. The applied magnetic field lifts the resistive heater 12out-of-plane by bending the cantilevered supports 16, rotating themabout the bending joint 34 and away from the substrate 14 at an angle θ(FIG. 2I). Other methods to apply the magnetic field H_(ext), such aselectromagnets, may be employed. Finally, a chip 72 including thesubstrate 14 and the thermal sensor 10 is rinsed in deionized water anddried.

The adhesion between the (preferably gold) metal beam of the supports 16and the polyimide support layer is significant, as the adhesion helps toreduce the likelihood of separation during the three-dimensionalassembly step (FIG. 2I). One preferred method of improving adhesion isto use a Cr layer as an adhesion layer as described above and also totreat the polyimide layer 62 by using O₂ reactive ion etching (RIE)before the deposition of the metal (gold) layer 66. By employing the RIEtreatment, a hydrophilic structure is created on the polyimide layer 62surface that enhances adhesion. Cr is preferred for the adhesion layer,as other materials, such as titanium, though having good stability inchemical etchants and higher electrical resistivity, may allow the metallayer 66 to peel off from the polyimide 62 during the three-dimensionalassembly.

Both the resistive heater 12 and the supports 16 preferably haverelatively small frontal areas. The momentum thus imparted by fluid onthe thermal sensor 10 is minimal. Accordingly, a preferred thermalsensor 10 can withstand airflow with a mean stream velocity lower than 5m/s without being damaged. However, at such velocity the supports 16 mayvibrate. Because the thermal sensor 12 is preferably at least partiallyimmersed in the velocity boundary layer, it is possible to developsensors that can withstand high flow velocity by lowering its height andfully immersing the thermal sensor in the boundary layer.

For certain applications, it may be advantageous to strengthen thebending region 34 (hinge) of each of the supports 16 so that the thermalsensor 10 can operate at (i.e. withstand) high flow speed of the fluidmedia 22. FIGS. 3A–3C shows a step in the preferred process including anadditional post-sacrificial layer 60 release electroplating step. Inthis step, a nickel layer 74 is selectively electroplated at the benthinges 34 to strengthen the hinges. Other electroplated materials mayalso be used. Referring first to FIG. 2G, before the removal of thecopper sacrificial layer 60, the photoresist 70 is spun on and patternedto prevent the electroplated nickel 74 from growing on the resistiveheater 12. Thus, the only metals preferably exposed at this stage arethe (gold) bending regions 34 and the electrical leads of the supports16. The sample chip 72 is then put in a copper etchant until thesacrificial layer 60 including copper is completely undercut (FIG. 2H).Afterwards, the sample chip 72 is removed from the etchant solution,thoroughly rinsed in deionized water, and immediately placed into anickel plating bath.

The external magnetic field H_(ext) is then applied, preferably in adirection normal to the substrate 14 (FIG. 3A) in parallel to lift thesupports 16 out-plane. Next, the nickel layer 74 is locallyelectroplated onto the hinge 34 (FIG. 3B). A preferred plating time maybe between 5–30 minutes, for example, at preferably between 1 and 1000mA/cm² per device, depending on the size of the chip 72. This provides anickel layer 74 at the hinge 34, preferably of a few μm in thickness,though this may be higher (on the order of tens of microns, forexample). After plating is completed, the sample chip 72 is removed fromthe plating bath and the resist 70 is stripped (FIG. 3C). FIG. 4 showsan embodiment of the thermal sensor 10 after release and assembly.

Alternatively, the thermal sensor 10 may be formed without fabricationof the polyimide supports 30, so that the supports 16 include only metalcantilever beams 28 (and the ferromagnetic material 68 used forassembly, unless it has been removed), and the non-silicon thin film 40used for the resistive heater 12 is exposed to fluid media on bothsides. A scanning electron microscope (SEM) micrograph of such a thermalsensor is shown in FIG. 5.

In operation of the thermal sensor 10, preferably as part of the chip72, the supports 16, serving as electrical leads, are coupled, directlyor indirectly, to the source 76. For example, the sensor 10 may behooked up to a constant current or constant temperature driving circuit76 via a pair of conductive paths 77. The output of the driving circuit76 goes through a signal conditioner 78 (for amplification andlinearization, for example) and finally to an A/D converter and readout79 for determining fluid flow, for example. The output may also bemeasured by a multimeter.

For example, FIG. 7 shows a nonlimiting example of a basic constantcurrent driving circuit 80 for analyzing constant current response ofthe thermal sensor 12, including a constant current source 82 in serieswith a resistor R1, and the thermal sensor 10 as resistor R_(s). As onenon-limiting embodiment, an LM334 current source 82, in conjunction witha diode to reduce temperature dependence on the circuit, may be used forthe source. Due to a positive TCR, the voltage across the resistiveheater 12 decreases as the flow rate increases. The sensitivity ofoutput voltage with respect to air velocity increases with increasingoverheat ratio because of the increase in temperature difference betweenthe resistive heater 12 and the fluid media 22.

FIG. 8 shows a nonlimiting example of a second basic circuit 90, usedfor constant temperature mode operation. In constant temperature modeoperation, the resistance of the thermal sensor R_(anm) is kept constantby a feedback op-amp circuit. The second basic circuit 90 includes aWheatstone bridge, three legs of which contain resistors R₁, R₂, R₃,respectively, and one leg including the thermal sensor R_(anm). Thebridge circuit is balanced when R_(anm)R₂=R₁R₃. Resistor R₃ is avariable resistor used to set an overheat ratio.

In addition to the chip 72 having an individual thermal sensor 10, achip according to an alternative embodiment of the present invention canalso be realized in a large array format for distributed flow sensing.For example, FIG. 8 shows a triple-point thermal sensor array 100including first, second, and third thermal sensors 10. Thethree-dimensional assembly process described above allows, among otherbenefits, assembly of multiple thermal sensors 10 in parallel using aglobally applied magnetic field. However, the array 100 of thermalsensors 10 need not be parallel, co-planar, or even disposed on the samesubstrate 14. For example, a plurality of thermal sensors 10 havingresistive heaters 12 disposed orthogonally to one another can be used toprovide flow rates in three dimensions, and thus determine orientationof fluid flow.

Furthermore, by integrating a strain gauge into a base of the polymersupport structure of the present thermal sensor, a tactical sensor canbe produced as well. Arrays of such tactical sensors are possible formeasuring water flow and airflow.

While various embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions, and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions, and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

Various features of the invention are set forth in the appended claims.

1. A thermal sensor for sensing fluid flow provided on a substrate,comprising: a resistive heater including a thin film metal, saidresistive heater being suspended above the substrate by supportsconnected at one end to said substrate; each of said supports comprisinga ductile metal beam bent along a joint near ends connected to saidsubstrate; said supports being raised at a sufficient angle relative tothe substrate to create a clearance between said resistive heater andsaid substrate.
 2. The thermal sensor of claim 1 wherein the resistiveheater is raised with respect to said substrate above an immediatevelocity boundary layer of the fluid media over said substrate.
 3. Thethermal sensor of claim 1 wherein each of said supports furthercomprises a polymer support beam.
 4. The thermal sensor of claim 1wherein the thin film metal comprises a metal having a high temperaturecoefficient of resistance (TCR).
 5. The thermal sensor of claim 1further comprising a polymer support overlapping the thin film metal. 6.The thermal sensor of claim 1 wherein said supports comprise a pair ofsupports supporting first and second opposing ends of said resistiveheater, each of said pair of supports being bent along a joint near theends of said pair of supports connected to said substrate.
 7. Thethermal sensor of claim 1 wherein said supports comprise a pair ofsupports providing structural support for said resistive heater and anelectrical connection to said resistive heater.
 8. The thermal sensor ofclaim 7 further comprising: a source for supplying current to saidresistive heater via said pair of supports.
 9. A microscale anemometer,comprising: a thin film metal heater supported above a substrate andspanning a pair of ductile metal supports having a portion extendingfrom the substrate; a source for supplying current to said thin filmheater, the supports being coupled to said source via a portion of saidsupports extending above the substrate and in a direction out of planewith the portion of the supports extending from the substrate; saidheater being raised from said substrate to create a clearance for flowof a fluid media between said heater and the substrate; said source,heater, and supports forming part of a circuit.
 10. A chip comprising: asubstrate defining a plane; a resistive heater including a thin-filmmetal; a pair of supports connected to said substrate and said heater,said supports raising said heater with respect to said substrate tocreate a clearance for flow of a fluid media between said substrate andsaid heater; said pair of supports and said heater being electricallyconnected; said heater and said pair of supports being adhered to oneanother.
 11. The chip of claim 10 wherein said heater further comprisesa polymer support.
 12. The chip of claim 11 wherein said heatercomprises at least one of nickel, platinum, and chrome.
 13. The chip ofclaim 10 wherein said heater and said pair of supports are manufacturedusing surface micromachining.
 14. The chip of claim 10 furthercomprising: a plurality of additional heaters and supports supportingsaid heaters over said substrate to create an array of sensors.
 15. Thechip of claim 14 wherein said array of sensors is co-planar.
 16. Thechip of claim 14 wherein said sensor and said additional sensors areeach disposed orthogonally to one another.
 17. A thermal sensor forsensing fluid flow provided on a substrate, comprising: a thin filmmetal resistive heater suspended above the substrate by supportsconnected at one end to said substrate; a polymer support for at leastone of said supports and said resistive heater; each of said supportscomprising a metal beam; said supports being raised at a sufficientangle relative to the substrate to create a clearance between saidresistive heater and said substrate.
 18. The thermal sensor of claim 17wherein said polymer support is adhered to said resistive heater. 19.The thermal sensor of claim 17 wherein said polymer support is adheredto said supports.
 20. The thermal sensor of claim 17 wherein saidpolymer comprises polyimide.
 21. A microscale thermal sensor for sensingfluid flow provided on a substrate, comprising: a thin film metalresistive heater suspended above the substrate by metal supportsconnected at one end to said substrate; said supports being raised at asufficient angle relative to the substrate to create a clearance betweensaid resistive heater and said substrate; said thin film heater having athickness between hundreds of angstroms and several micrometers.
 22. Themicroscale thermal sensor of claim 21 wherein said thin film heater isbetween 10 micrometers and 200 micrometers in length.
 23. A chipcomprising: a substrate defining a plane; a thin-film metal resistiveheater; a pair of supports connected to said substrate and said heater,said supports raising said heater with respect to said substrate tocreate a clearance for flow of a fluid media between said substrate andsaid heater; said pair of supports and said heater being electricallyconnected; further comprising a plurality of additional heaters andsupports supporting said heaters over said substrate to create an arrayof sensors.
 24. The chip of claim 23 wherein said array of sensors isco-planar.
 25. The chip of claim 23 wherein said sensor and saidadditional sensors are each disposed orthogonally to one another.